use a diode (or bridge rectifier), then RC filter to turn your AC audio signal into a DC voltage (similar to a power supply), duplicate for the other audio signal, then pass both DC voltages into a comparator (op-amp). throw in op-amps to handle levels & impedance, done.
you could even use the comparator output to drive the motor direction (through an h-bridge, probably) so it would be self-centering.

Hi Eric, my name is Eric as well. I have the same issue as you. Coworker doesnt believe the earth is round and sadly thinks the moonlanding isnt real. How did you deal with this? Been 6 months in and its driving me bonkers.

Sadly I have to deal with said person a few times a month on a high up level. Maybe so but this person I know truly believes we didnt land on moon at least. Went full nuke mode at a office party. Thinks it's all stage by movie directors.

Mueller hasn’t found anything in 2 years...what evidence has been brought forward? All I see is opinions being written of the man, racist this, disgusting that. That’s all fine and dandy but where is the EVIDENCE of wrongdoing? Anyone help me out?

What I don’t quite understand is this: say you have a huge lump of lead that’s not quite large enough to be a black hole. Then you throw a small lump of lead at it, enough to make it a black hole, does the whole thing just suddenly shrink to a single point because of this? What’s the mechanism by which that happens?

That's a decent way of putting it. I like to say that if a Phoenix with the head of Nic Cage came down from the sky shooting fire from its ass I'd be a believer. There's tangible proof. but no existing religion has anything I'd remotely consider to be credible evidence.